1. Field of the Invention
The present invention relates to a scroll type pump, and, more particularly, to a scroll type pump, such as a vacuum pump, which is of an oil free type and suitable for attaining high performance and high reliability.
2. Statement of the Related Art
A scroll type fluid pump include a housing, a pair of scroll members each comprising an end plate and a spiral wrap means projecting from one surface of the end plate. Both wrap means are engaged with each other to make a plurality of line contacts between them, and a driving mechanism including a drive shaft is connected to one of the scroll members to effect orbital motion thereof relative to the other (fixed) scroll member while rotation of the orbiting scroll is prevented.
If a scroll type fluid apparatus is operated in an oil free system, an orbiting scroll is heated by the heat generated due to compression of the fluid, and if the machine is operated as a vacuum pump, it becomes impossible to expect the transfer of compressional heat and the cooling by the lubricating oil which has been conventionally employed. At the same time, since the orbiting scroll is placed in vacuum, it becomes impossible to expect the cooling to be obtained by the dissipation of heat through convection. Therefore, the temperature of the orbiting scroll tends to become high. On the other hand, an outer wall of the fixed scroll is generally disposed in an atmospheric environment and the cooling or dissipation of heat from other surface of the outer wall through convection can be made relatively effectively. Accordingly, the fixed scroll can be cooled relatively effectively and the temperature of the fixed scroll can be kept relatively low. Consequently, the temperature of the orbiting scroll becomes higher than that of a fixed scroll. As a result, distortion occurs in a wrap of the orbiting scroll due to thermal expansion.
In a scroll compressor for refrigeration or airconditioning, partly due to the fact that the suction gas is cold, the rise in temperature in the vicinity of a center portion of the wrap is greater than that in the other outer peripheral portion, with the result that the degree of distortion experienced by the wrap is large. There has accordingly been a problem in that the gap between the wraps becomes large at the center portions thereof which results in leakage of fluid, thereby causing a decline in the compression performance.
As a countermeasure against this problem, a scroll type fluid displacement apparatus is disclosed in the specification of U.S. Pat. No. 4,490,099. In this apparatus, the center portions of the wraps are made slightly thicker than the remaining portions thereof and leakage of fluid in the center portions of the wraps is prevented, thereby improving the performance of the apparatus.
In a scroll type vacuum pump, the suction side assumes a vacuum in which the flow rate of a gas is small. Therefore, in case of the orbiting scroll since substantially no heat dissipation occurs, the temperature in an outer peripheral portion thereof also rises and thus thermal expansion takes place. The farther the outer peripheral portion is from the center, the greater is the amount of this thermal expansion. If the orbiting scroll undergoes thermal expansion, an outer wall surface and an inner wall surface of the wrap are deformed radially to approach the inner wall surface of the wrap of the fixed scroll and to become remote from the inner wall surface of the wrap of the fixed scroll. According to the above-described prior art, however, since sufficient consideration is not paid to the radial thermal expansion of outer peripheral portions of the wraps of the orbiting scroll and the fixed scroll, there has been a problem in that the side wall surfaces of the wraps are brought into contact with each other particularly in the outer peripheral portions thereof and vibrations occur as a result of that contact.